1. Field of the Invention
The present invention relates generally to methods of forming glass articles, and more specifically to methods of forming titania-doped silica glass articles suitable for use in extreme ultraviolet lithography applications.
2. Technical Background
The desire for faster, more powerful integrated circuits has led to rapid development of advanced ultraviolet photolithographic methods. The performance of an integrated circuit increases as the feature size decreases, since a decrease in feature size allows more circuitry to be put on a chip of a given size, and reduces the power needed for operation. The feature sizes obtainable in a photolithographic process depend on the wavelength of radiation used in the exposure step; shorter wavelengths enable smaller feature sizes. As such, there has been a trend toward shorter wavelengths in photolithographic processes. Currently, photolithography systems based on wavelengths as low as 193 nm and 157 nm are being developed for commercial use.
In order to further decrease the minimum feature size obtainable in photolithographic processes, it has been suggested to use extreme ultraviolet (EUV) radiation, for example, having a wavelength of about 13 nm. The use of EUV radiation in photolithography, while greatly reducing feature size, forces a radical departure in design from currently used photolithographic apparati. First, no currently practical condensed materials are sufficiently transparent to EUV radiation to be used as windows, lenses or photomasks. As such, any manipulation of the radiation must be performed using reflection. Mirrors constructed from alternating layers of molybdenum and silicon deposited on low thermal expansion substrates have been used as reflecting focusing mirrors, collimators, and photomasks in EUV apparati.
Titania-doped silica glass is conventionally used as the low thermal expansion substrate in EUV lithography mirrors. In one conventional method, high purity titania-doped silica glasses are formed by deposition of titania-doped silica soot formed by flame hydrolysis of silicon- and titanium-containing precursors (e.g., octamethylcyclotetrasiloxane and titanium isopropoxide). The soot is deposited in a revolving collection cup in a refractory furnace at a temperature high enough to consolidate the soot into a glass body. The amount of titania is adjusted so that the glass body has a coefficient of thermal expansion of about 0 ppb/° C. at a desired operating temperature. Titania-doped silica made using this method tends to have striae formed therein. Striae are periodic inhomogeneities in the glass which adversely effect many properties of the glass. For example, the striae cause an inhomogeneity in thermal expansion in the glass, causing reflective optics made therefrom to have less than optimal thermal properties. The striae can also impact the surface finishing of the glass, making it difficult to form the extremely smooth surfaces needed in EUV lithography elements. The striae also cause optical inhomogeneities, which make the glass unsuitable for use in many optical transmission elements (e.g., lenses, windows, prisms).
It would be advantageous to provide new methods for manufacturing a titania-doped silica glass having a low thermal expansion, a high homogeneity, good optical properties and a precisely controlled composition.